Magnetic susceptibility and electric conductivity of marine surficial sediments by benthic electromagnetic profiling

Distribution, accumulation and diagenesis of surficial sediments in coastal and continental shelf systems follow complex chains of localized processes and form deposits of great spatial variability. Given the environmental and economic relevance of ocean margins, there is growing need for innovative geophysical exploration methods to characterize seafloor sediments by more than acoustic properties. A newly conceptualized benthic profiling and data processing approach based on controlled source electromagnetic (CSEM) imaging permits to coevally quantify the magnetic susceptibility and the electric conductivity of shallow marine deposits. The two physical properties differ fundamentally insofar as magnetic susceptibility mostly assesses solid particle characteristics such as terrigenous or iron mineral content, redox state and contamination level, while electric conductivity primarily relates to the fluid-filled pore space and detects salinity, porosity and grain-size variations. We develop and validate a layered half-space inversion algorithm for submarine multifrequency CSEM with concentric sensor configuration. Guided by results of modeling, we modified a commercial land CSEM sensor for submarine application, which was mounted into a nonconductive and nonmagnetic bottom-towed sled. This benthic EM profiler Neridis II achieves 25 soundings/second at 3-4 knots over continuous profiles of up to hundred kilometers. Magnetic susceptibility is determined from the 75 Hz in-phase response (90% signal originates from the top 50 cm), while electric conductivity is derived from the 5 kHz out-of-phase (quadrature) component (90% signal from the top 92 cm). Exemplary survey data from the north-west Iberian margin underline the excellent sensitivity, functionality and robustness of the system in littoral (~0-50 m) and neritic (~50-300 m) environments. Susceptibility vs. porosity cross-plots successfully identify known lithofacies units and their transitions. All presently available data indicate an eminent potential of CSEM profiling for assessing the complex distribution of shallow marine surficial sediments and for revealing climatic, hydrodynamic, diagenetic and anthropogenic factors governing their formation.沿海与大陆架体系中表层沉积物的分布、堆积与成岩作用遵循一系列复杂的局域过程，形成空间异质性极强的沉积体。鉴于大陆边缘的环境与经济重要性，学界对创新型地球物理勘探方法的需求日益增长，以期突破仅依靠声学特性表征海底沉积物的局限。一种基于可控源电磁（CSEM）成像的新型底栖剖面测量与数据处理方法，可同步量化浅海沉积体的磁化率与电导率。这两种物理性质的本质差异在于：磁化率主要反映固体颗粒的特征，例如陆源物质或铁矿物含量、氧化还原状态与污染水平；而电导率则主要与充填流体的孔隙空间相关，可探测盐度、孔隙度与粒度变化。我们针对采用同心传感器配置的海底多频可控源电磁系统，开发并验证了层状半空间反演算法。基于模拟结果的指导，我们将一款商用陆上可控源电磁传感器改造为适用于海底环境的版本，并将其安装在非导电、无磁性的底拖式滑橇中。这款名为Neridis II的底栖电磁剖面仪，可在3至4节的航速下，在长达百公里的连续测线上实现每秒25个电磁测深点的采集。磁化率通过75Hz的同相响应计算得到（90%的信号来自表层50cm深度），而电导率则由5kHz的正交（异相）分量推导而来（90%的信号来自表层92cm深度）。来自伊比利亚西北陆缘的典型调查数据表明，该系统在滨海（约0~50米）与浅海架（约50~300米）环境中均展现出优异的灵敏度、功能性与稳定性。磁化率-孔隙度交会图可成功识别已知的岩相单元及其过渡界面。当前所有可用数据均表明，可控源电磁剖面技术在表征浅海表层沉积物的复杂分布，以及揭示控制其形成的气候、水动力、成岩与人为因素方面，具备极高的应用潜力。